This paper addresses an ongoing experience in the design of an artificial agent taking decisions in a social simulation (more precisely, a role playing game) populated by human agents and by artificial agents. At first, we will present our current research context, an ongoing research project aimed at computer-based support for participatory management of protected areas (and more specifically national parks) in order to promote biodiversity conservation and social inclusion. Our applicative objective is to help various stakeholders (e.g., environmentalist NGOs, communities, tourism operators, public agencies) to collectively understand conflict dynamics for natural resources management and to explore negotiation management strategies for the management of parks, one of the key issues linked to biodiversity conservation. Our approach combines techniques such as: distributed role playing games (serious games), geographical information systems, support for negotiation between players, and insertion of various types of artificial agents (virtual players, decision making agents, assistant agents). In this paper, we will focus on the design of the decision making agent architecture for the park manager agent, the rationales for his decision and how he takes into account the preferences/votes from the players of the game and may justify/explain his decisions.